Recently, interests in energy storage technologies have been increasingly grown, and efforts to research and develop electrochemical devices have been gradually materialized while the application of the energy storage technologies is expanded to mobile phones, camcorders, notebook PCs, and even to electric vehicles.
There emerges an interest in the development of rechargeable secondary batteries among these electrochemical devices, and, particularly, lithium secondary batteries developed in the early 1990's are spotlighted because the lithium secondary batteries are advantageous in that they have higher operating voltage and significantly higher energy density.
After a positive electrode collector and a negative electrode collector are respectively coated with materials, in which a positive electrode active material formed of a lithium-containing transition metal oxide or a carbonaceous negative electrode active material capable of intercalating and deintercalating lithium ions as well as selectively a binder and a conductive agent is mixed, to prepare a positive electrode and a negative electrode, a lithium secondary battery is generally prepared by laminating the positive electrode and the negative electrode on both sides of a separator to form an electrode assembly having a predetermined shape and then inserting the electrode assembly and a non-aqueous electrolyte solution into a battery case. Herein, in order to secure performance of the battery, the battery is almost inevitably subjected to formation and aging processes.
The formation process is a process of activating the secondary battery by repeating charge and discharge after the assembly of the battery, wherein lithium ions deintercalated from the lithium-containing transition metal oxide used as the positive electrode move and are intercalated into the carbonaceous negative electrode active material used as the negative electrode during the charge. In this case, the highly reactive lithium ions react with the electrolyte solution to form compounds, such as Li2CO3, Li2O, and LiOH, and these compounds form a solid electrolyte interface (SEI) on the surface of the negative electrode.
The aging process stabilizes the above-described activated battery by leaving the battery for a predetermined period of time.
Recently, as high-temperature and high-voltage working of the lithium secondary battery is required, gas generation is increased during high-temperature storage while an electrolyte solution decomposition reaction caused by an oxidation reaction between the electrolyte solution and the positive electrode is accelerated, and thus, there is a limitation in that life characteristics are degraded.
In order to address this limitation, there is a need to develop a method of preparing a lithium secondary battery which may reduce the gas generation during high-temperature storage and may simultaneously prevent the reduction of the cycle life characteristics.